1. Field of the invention
The present invention concerns positive power ophthalmic lenses and more particularly lenses of this kind in which the power is relatively moderate, for example in the order of 2 to 9 diopters.
2. Description of the prior art
A positive power ophthalmic lens has to satisfy numerous requirements.
The first and most important is that it should have a central optically active area as large as possible, it being understood that in this context there is meant by "optically active area", in the usual way, an area in which astigmatism and field curvature aberrations are sufficiently well corrected to provide satisfactory vision.
This optically active area generally covers all of the lens, its limited and constant diameter irrespective of the power of the lens being then equal to that of the geometrical contour of the lens.
In the case of a high positive power lens a peripheral merging area is provided around the optically active area, between it and the geometrical contour of the lens.
A second requirement is that the thickness at the edge of the lens should have a particular value, among other reasons for mechanical strength and to enable fitting to a frame.
However, a third requirement that is just as important as the previous two is that the thickness at the center of the lens should be as small as possible to ensure wearer comfort, and in particular that the lens has minimal weight.
These usual requirements relate to the geometry of the lens and its optical characteristics. They are usually complemented by an industrial requirement relating to the fact that for simple economic reasons it is desirable that the lenses be easy to manufacture directly by molding any appropriate synthetic material such as diglycol allyl carbonate, for example.
Currently, however, to meet user expectations it is conjointly desirable for the diameter of the geometrical contour of the lens to be as large as possible, for mounting in large frames, that is to say frames with large rings or surrounds.
Currently lenses with a diameter exceeding 70 mm are required to equip certain frames.
This requirement is contradictory with the previous requirements, however, or at least with those relating to the geometry of the lens and its optical characteristics, even for moderate powers.
In fact, it is not currently possible to fit large frames with moderate positive power ophthalmic lenses.
This applies equally to lenses in which both surfaces are part-spherical and lenses with at least one aspherical surface.
There is therefore at this time some demand for positive power ophthalmic lenses which are of large diameter, well corrected and relatively light in weight.
The present invention is based on the new discovery that by adhering to a certain relationship between the diameter of the optically active area of the lens and its power it is possible to arrive at a compromise satisfying all these requirements and therefore meeting the aforementioned demand.